This invention relates to an improved heat exchanger, and is particularly concerned with a multipass heat exchanger wherein the flow path on the shell side of the heat exchanger is designed to achieve minimum weight of the heat exchanger and minimum pressure loss.
This invention was made with Government support under Contract No. NAS3-25559 awarded by the United States Air Force. The Government has certain rights in this invention.
To approach maximum effectiveness, shell and tube heat exchangers often employ many passes of the shell side fluid across the tube bank in a cross-counterflow arrangement. FIGS. 1a and 1b of the drawings show two conventional design approaches. FIG. 1a shows a conventional 3 pass cross-counterflow shell and tube heat exchanger 10 employing a pair of baffles 12 and a pair of end headers 14. FIG. 1b is also a conventional design of heat exchanger 16 using multiple baffles 18 and multiple shell side warm fluid headers 20 to reverse shell side flow, together with end headers 22.
As illustrated in FIG. 2, where large capacity at maximum performance is desired, a design arrangement 24 is conventionally used comprised of a number of parallel exchanger units 26, each of a type similar to FIG. 1b and having a plurality of baffles 28 and a plurality of shell side headers 30 for flow reversal. It is noted that a large total number of hot fluid headers and of flow reversals is embodied in this arrangement.
Maximum performance is indicated by high transfer of heat per unit volume with small pressure loss and a low temperature difference driving the heat transfer. A measure of the weight efficiency of the design is the "wrap factor", defined as the weight of the shell and baffles, compared with the weight of the heat transfer tubes. Light weight designs are characterized by low values of this ratio or wrap factor, and are of importance in aerospace applications.
U.S. Pat. No. 4,501,320 to Lipets, et al discloses a multiflow tubular air heater employing a two-pass heat exchange concept embodying a Z-type bypass conduit. Other illustrative prior art heat exchangers are disclosed in U.S. Pat. Nos. 2,002,763; 2,327,491; 2,487,626; 3,180,406; and 4,559,996.
It is accordingly an object of the present invention to provide a shell and tube heat exchanger having minimum weight and minimum pressure loss.
Another object is to minimize the header weight, or wrap factor, of a system of parallel multipass exchangers of the type shown in FIG. 2, and to reduce the pressure loss of the fluid flowing across the outside of the tubes.
A further object is the provision of an arrangement of multiple exchangers wherein there is no flow reversal and hence no flow reversal headers, and the shell side fluid passes through the exchangers in a straight through fashion, with minimum pressure loss.